I just purchased a W116 300SD. When I turn the key the glow plug light stays on and it does not go out until I start the car. How do I fix this? Does the glow plugs stay on until the car is started or do they go out at a predetermined time? I have just been guessing how long to wait before I try to start the car. Thanks in advance for any help. I also have a W115 240D but it has a gorilla knob so I don't have any experience with a glow plug light.